A grapefruit-size probe could help solve mysteries right beneath our feet


In "Too Hard for Science?" I interview scientists about ideas they would love to explore that they don't think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as particle accelerators as big as the sun, or they might be completely unethical, such as lethal experiments involving people. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard for Science?" suggests that nothing might be impossible.

The scientist: David Stevenson, George Van Osdol Professor of Planetary Science at the California Institute of Technology.

The idea: The most distant spacecraft humanity ever launched, Voyager 1, is now more than 17 billion kilometers from Earth, but our subterranean missions have only reached a little more than 12 kilometers into the Earth. Stevenson would like to send a probe to the Earth's core, a grapefruit-sized communication device that would ride tons of molten iron down through a crack blasted in the planet's surface, with the metal closing up the hole behind the machine.

"It would enable us to find out what the core is made of, its temperature and perhaps something about its dynamics — the vigor of the fluid motions that create Earth's magnetic field," Stevenson says. "We might imagine we know approximate answers to these questions, but the history of science suggests that there are surprises there, or anywhere we've not been to before."

The crack would need to be about 30 centimeters wide and several hundred meters long and deep to start with. The energy needed to create it would be equivalent to a few megatons of TNT, an earthquake of magnitude 7 on the Richter scale, or a nuclear weapon such as those currently possessed by many nations.

The journey should take at least 100,000 to 10 million metric tons of iron, or roughly between an hour and a week of Earth's total iron foundry production, Stevenson says. All told, the probe should travel about 18 kilometers per hour, taking about a week to reach the core 3,000 kilometers below, he calculates. It would measure temperature, electrical conductivity and chemical composition, transmitting its findings back to the surface using high-frequency sound waves.

Although the plan might sound far-fetched, he argues it's modest in comparison to the demands of space travel.

The problem: Stevenson first proposed this mission to the Earth's core in the May 15, 2003 Nature, but no one has seriously pursued the idea.

"It is probably impractical," Stevenson says. "I proposed my idea somewhat tongue in cheek, in part to get people to think about something that normally gets insufficient attention — traveling down instead of traveling up. But I did make a serious attempt to identify the basic physical principles correctly. It is possible to have an idea that has the right physics, more or less, but is impractical because of the size of what is needed, which translates to cost, or the likelihood of failure when first attempted. People sometimes forget how frequently early rockets failed to launch spacecraft."

The solution? Scientists may not be journeying to the core of the Earth anytime soon, but they might journey to the mantle within the next decade, aiming to go deeper than anyone has ever delved before. Researchers want to resurrect Project Mohole, which 50 years ago retrieved the first scientific core from the sea floor and sought to drill into the mantle, a project that eventually failed due to poor management that led costs to spiral out of control, explained Benoît Ildefonse at Montpellier 2 University in France and Damon Teagle of the University of Southampton in England in the March 24 Nature.

If funding can be found, drilling for a new Project Mohole could begin within the next 10 years and be completed within 15 years, retrieving samples of mantle only about 6 centimeters thick. To practice their deep drilling, Ildefonse and Teagle are chief scientists on a mission to get the first samples of the lower oceanic crust, Integrated Ocean Drilling Program Expedition 335, which started on April 13 and is due to finish June 3.


If you have a scientist you would like to recommend I question, or you are a scientist with an idea you think might be too hard for science, e-mail me at toohardforscience@gmail.com

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About the Author: Charles Q. Choi is a frequent contributor to Scientific American. His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time he has traveled to all seven continents. Follow him on Twitter @cqchoi.

The views expressed are those of the author and are not necessarily those of Scientific American.